1. Field of the Invention
The present invention relates to a global positioning system (GPS) and GPS receiver, more particularly, to a GPS and GPS receiver that compensate a phase inversion of a navigation message and conduct a phase integration over a long period to enable positional information to be provided even if a signal intensity of a received GPS signal is low.
2. Description of the Prior Art
A conventional GPS receiver is intended to receive signals at an open site and is significantly restricted in its point of use. Therefore, even if user carries a portable GPS receiver, when user is at a site where reception conditions are not good, such as in a store or at the center of a city, user is required to go out of the store or move to an open site. Thus, a technique has been proposed that, even if a signal intensity of a received GPS signal is low, enables a positioning computation by subjecting the GPS signal to a signal processing such as a phase integration or power integration.
FIG. 6 illustrates an operation of a conventional signal processing. FIG. 6(B) shows an intermediate frequency signal obtained by down-converting the GPS signal. The intermediate frequency signal is a signal in which a set of twenty phase modulation signals referred to as pseudonoise signals is repeated as a unit, and the pseudonoise signal has a repetition period of 1 ms (reference symbols H and L in FIG. 6(A) denote two different phases). As shown in FIG. 6(A), the period of the pseudonoise signal is referred to as an epoch. Twenty pseudonoise signals constitute one bit in a 50-bps broadcast from a GPS satellite. A bit of the navigation message broadcasted by the GPS satellite is represented by the phase inversion of the twenty pseudonoise signals. FIG. 6(B) shows a case where the navigation message is represented by 010. The navigation message includes information required for positioning computation such as details, orbit information, and delay time due to atmosphere of each satellite, health data of satellites, and so on.
In a conventional process for improving sensitivity, the intermediate frequency signal is divided into blocks each having a length not significantly exceeding 20 ms (referred to as phase integration blocks), each of the phase integration blocks is further divided into sub-blocks each having a length of 1 ms corresponding to one epoch, and then the sub-blocks are added together. As shown in FIG. 6(C), the operations of dividing the signal waveform into the period of 1 ms and of adding the divided waveforms together to form one piece of data are collectively referred to as a phase integration. The phase integration emphasizes the waveform of the pseudonoise signal to improve an SN ratio of the intermediate frequency signal. This is a basic principle of the process for improving sensitivity. A GPS computation unit performs on the phase-integrated intermediate frequency signal a correlation processing with a pseudonoise signal previously generated and retained. As a result, as shown in FIG. 6(D), a phase shift between the received pseudonoise signal and the generated pseudonoise signal appears as a peak position, which is used in the GPS positioning computation.
In a conventional approach for improving sensitivity, due to the phase inversion of the pseudonoise signal of the navigation message, a time of the phase integration that is expected to provide the effect of improving sensitivity is limited to be not more than 20 ms. Therefore, outputs obtained by subjecting outputs of the phase integral to the correlation processing are made to be unsigned through a power processing, a transformation into absolute value, or the like, and then added together to form one piece of data. This operation is referred to as a power integration. In the power integration, the correlation processing is required to be performed for each phase integration block. Since the correlation processing is computation-intensive, the power integration is also computation-intensive, and therefore consumes significant power.
The present invention is devised to solve such a problem, and an object of the present invention is to provide a GPS and GPS receiver that compensate a phase inversion of a navigation message and perform a phase integration over a long period to enable positional information to be provided even if a signal intensity of a received GPS signal is low. Furthermore, another object of the present invention is to provide a GPS and GPS receiver that can provide a higher sensitivity than a conventional technique for a same integration time, requires a less integration time than a conventional technique in order to provide the same level of the effect of improving sensitivity, and reduces electric power consumption.
In order to solve the above-described problem, a GPS receiver according to the present invention comprises a high frequency unit for frequency-converting a received GPS signal to output an intermediate frequency signal, an A/D converter unit for A/D converting the intermediate frequency signal to output intermediate frequency signal sample data, a memory unit for storing the intermediate frequency signal sample data for a predetermined period of time, the data being associated with a time series, a communication unit for communicating with a base station, an integration unit for subjecting the intermediate frequency signal sample data stored in the memory unit to a phase correction based on phase inversion information of a navigation message supplied from the base station, and then performing a phase integration on all of the intermediate frequency signal sample data for the predetermined period of time to output a result of the phase integration, and a GPS computation unit for computing positional information based on the result of the phase integration.
Moreover, a GPS according to the present invention comprises the GPS receiver described above, and a base station for providing the phase inversion information of the navigation message and information for supporting a positioning computation to the GPS receiver.
With the GPS and GPS receiver according to the present invention, the data obtained by sampling the intermediate frequency signal of the GPS signal can be phase-integrated, from its beginning to its end, over a period of time in seconds that is significantly longer than a conventional time of several tens milliseconds. As a result, the SN ratio of the signal can be effectively improved.
Since the power integration is not performed, the process for improving sensitivity does not require any correlation computation. As a result, the computation load involved in the process for improving sensitivity can be significantly reduced compared with the case where the power integration is performed, and therefore, power consumption can be reduced.
Since the GPS receiver is not required to acquire the phase inversion of the pseudonoise signal of the navigation message, the process for improving sensitivity can be performed at any site where the GPS receiver can communicate with the base station.